Converter lets you convert these file formats:
avif
bmp
gif
heic and heif
jpeg/jpg
jxl
png
tif/tiff
webp
Into one of these formats:
avif
bmp
gif
heic and heif
jpeg/jpg
jxl
pdf
png
tif/tiff
webp
Pretty comprehensive on the ol’ image front (as you’d expect for something powered by ImageMagick) with a very simple, straightforward GUI.
...
The app is also available to install from the AUR, if you use a distro that can access it.