To comply with the upload limits of sm.ms, I need to compress the images before uploading, and then I found a project that can handle batch processing and has a Linux build:
Caesium is an image compression software that helps you store, send and share digital pictures, supporting JPG, PNG, WebP and TIFF formats. You can quickly reduce the file size (and resolution, if you want) by preserving the overall quality of the image.
It's quite useful and can basically compress more than half: