No idea how to do it with a phone. No matter how I tried I got crap photos with my phone. And believe me I tried, for 3 years. I recently got a good used camera and my photos have improved 100%. You can get a polarising filter for a camera, which is specifically meant to remove glare.

Purchase number 2? a tripod.

Another vote for using a tripod. A decent weight tripod coupled with a remote release and locking the mirror up if you can, will increase sharpness substantially. It also helps you get everything lined up and square vs hand held shooting.

Dont forget the camers lens on the iphone will distort your image no matter what and there will be convergence.

Okay with that out of the way you can fix the head on problem with a tabletop tripod. And just place your painting on a desktop easel at the end of the table. Dont use the flash or hdr! Use ambient lighting preferable from a window so the painting is facing the window preferably on an overcast day. Got nets for your windows. They make great diffusers! As for the glare. You can experiment with a linear polariser piece of glass. No need for a circular polarizer. These are very cheap on ebay, and just hold it in front of your camera. If not as a quick solution you can hold a pair of sunglasses in front of the camera. This may work. Just experiment a bit.

Of course a good camera with a fast prime is very hard to beat in these situations. But ive seen a lot of working pro photographers switch to the latest phone cameras(for convenience) that are now outresolving(some are 24mp) a lot of the older cameras as they are so convenient. So i’m with you on that. Scott Kelbys done some fantastic work with an iphone. Its often about the user not the tool. Best of luck buddy!

I have described my method of photographing my paintings so many times, I finally decided to create an “article” that I can merely copy, and post. Here it is:

Photographing an oil painting, for show, or for keeping a record.
It is a good technique to photograph your oil painting after you have applied a final varnish, but before you place it in a frame. Most galleries, or art competitions are not interested in seeing the art within a frame, when you submit your work.

When photographing your painting, one of the most important things is to keep all direct light off the surface. Direct light can cause a reflection, and it is these reflections that create the “glare”, and “hot spots” that are so often associated with amateur photography.

Such sources as the sun, flash from your camera, an overhead, interior lamp, and the sky [even on an overcast day] are all sources of direct light, and they should be kept from directly illuminating the surface of the painting.
I use a couple of very inexpensive, digital cameras, including one which is within my cell phone, and I seldom bother using a tripod.

To photograph my finished, varnished painting, I lay it face up, on the floor of one of my outdoor, covered patios, on a bright, sunny day. I straddle my painting with my feet, with one foot on the left, and one foot on the right of my painting, holding my camera in my hands, and aiming it straight down at my painting. A covered patio, a picnic ramada, a gazebo are all appropriate structures within which to lay your painting for photographing. They are covered, and the only light source is that which happens to be bouncing around inside the structure—there is NO DIRECT LIGHT!

I turn off the flash on my camera, so I can use the available light, only. I set my camera for as full a ZOOM setting as I can, while still being able to keep the painting within my viewer window. Setting a camera on Zoom helps to reduce, or eliminate the “barrel-shape” condition that seems so common with today’s digital cameras—even the more expensive ones. In fact, quite often the more expensive ones exhibit MORE of this barrel-shape effect than some of the less expensive models. It is really a LENS DISTORTION, created by the wide-angle lenses with which they equip modern, digital cameras.

Holding the camera in my hands, and up nearly to my chin (remember I am ZOOMED), I aim the camera straight down at my painting, I press the “focus” button (usually holding the “expose” button down halfway), I take a deep breath and let about half of it out, and I hold my breath while holding the camera as steady as possible while pressing the button to expose the photo.
I usually snap about 3 photos, while I’m there, but I generally am able to use any one of the three. I download them into Photoshop, and straighten one out by rotating it, crop it, and make minor color corrections, using “Curves”, or “Levels” tools. I save the corrected image, and I usually print out about 9 photos, 4” x 6” to keep in my record file.

Hire a professional.

I’m a pretty good amateur photographer. I’ve shot weddings, industrial catalog photos–got paid to get on stage with Bangles once, and take pictures of them for a charity event they headlined.

I could do my own photography.

But I don’t.

Hire a pro. That’s one headache you don’t need.

Focus on painting.

Thanks for the suggestions. I just got a new iPhone 8 Plus, which has their best camera with optical zoom, which hopefully will help as well. I also have a very nice LUMIX digital camera that I’ve never actually used to photograph paintings, and perhaps it will work best of all. I take all of my photos these days with my iPhone since I always have it with me and it takes great shots most of the time.

What size images does your phone produce?

I print up to 16 x 20, and I want super-sharp images at whatever size I’m printing.

My phone can’t produce anything close to the resolution I want.

My files, made by high-resolution scans direct from my paintings, average about 100mb per image.

For painting up to 11×14 in size, I use a flat bed scanner.