PrivateEye

Description
PrivateEye can read Sprite files, JPEGs, GIF files, PNGs, DrawFiles and ArtWorks formats, and can convert them to Sprite format. It can rotate JPEGs losslessly, and offers unlimited combinations of filtering effects that can be applied to files once they have been converted to Sprite format. It also offers image caching, which speeds things up when switching between several images in turn. A major distinguishing feature is its ability to read JPEG metadata better than any other current RISC OS tool. The author strongly recommends that anyone using PrivateEye downloads and reads the program's comprehensive PDF format manual.

Source
Source code for PrivateEye is accessible via one of the riscos.info's source repositories.

Alternatives
Programs such as InterGif, Variations, Creator and Translator, SwiftJPEG, Spr2Png, Png2Spr and others can between them match most of PrivateEye's capabilities, but many of these older programs are no longer being actively developed.