Is participating in image-processing challenges allowed, if the language does not support images? If it is allowed, what shall the input format be?
Related question where this issue came up in the comments.
All languages (which support input via an arbitrary stream of bytes) technically support images. An image file (PNG, JPG, GIF, etc.) can simply be processed without builtin image manipulation functions or image libraries.
However, clearly it is impractical to expect code-golf answers to rewrite an implementation of PNG for every challenge. A popular image format that I've used and seen used on PPCG is Netpbm, which is extremely easy to parse; images can be converted to and from .p*m with Imagemagick (
convert) or a similar tool.
For challenge-writers: Unless the image format is integral to the challenge (such as Output a solid PNG from scratch), it is strongly recommended that you do not arbitrarily restrict image formats for input and output, so that all languages can reasonably participate.