I have downloaded a large set of images from the Universal Postal Union website. The UPU has been hosting images submitted by member countries since 2002. Although there are some significant gaps in some years due to some member countries being slow in providing images of their issues, the images are mostly high quality jpg or png. Most images are about 500 pixels wide.

If anyone is interested in designing illustrated albums, I can provide you with access (via a google drive link) to the images for your country of interest. Please message me if you are interested.