As mentioned in What is a Document? - scanned documents play a central role in Papermerge. Technically, scanned document can be stored in a plethora of file formats. Papermerge supports following, the most common ones:
PDF - Portable Document Format
TIFF - a convenient file container. TIFF files can contain inside several images
JPEG - popular image format
PNG - popular image format
Many scanners output their scanned documents as PDF, TIFF or JPEG.
Strictly speaking, JPEG and PNG are images. From the point of view of Papermerge - they are one page documents - it is assumed that JPEG contains scanned pages information. Many people nowadays use their mobile phones to capture documents.
Papermerge is not suitable to store pictures of pets, flowers, family portraits, trees, grass, selfies etc. That being said - you can upload any picture you want! Application won’t complain as long as image is in one of supported technical formats. Papermerge will try to OCR uploaded picture and will handle it as regular document. Just keep in mind that Papermerge is designed work with documents (even if they are images taken with your phone).