Due to real time engine specifications, a texture should have specific dimensions (multiple of 2) to display correctly in Augment. You can mainly use : 2048, 1024, 512, 256, 128, 64, 32 pixels.
The best texture proportions are square ones (2048 x 2048, 1024 x 1024, 512 x 512, etc ...). However, in Augment's engine you can use non square multiple of 2 dimensions, as in this tutorial : 1024 x 512 (or 2048 x 1024, 1024 x 256, etc...).
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If you don't have any image edition software, off-sized textures can also be used, as Augment platform automatically resizes them.

However please be aware that you may lose in quality compared to manual resize (with Photoshop or any other image editor).

When resizing to adapted dimensions, if you didn't crop the image for resizing, it will distort a bit the image, which is perfectly normal. It will still display correctly in 3D as long as you don't edit the UVs of your 3D model.

However please avoid distorting too much the map to avoid blurry display in the app.

/!\ If you upload only an image file (without 3D file) please do not distort the image when resizing, as the image isn't applied on 3D it will display the distortion /!\

Following is an example of picture, and the result of the resizing:
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​Before the resizing (1280 x 853 pixels) :

After resizing (1024 x 512 pixels) : 

You can see here the result in Augment, applied on a 3D model of an artwork (it is using a 3D file, not direct 2D image upload).

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To get other information regarding textures in Augment, check here.