Hi there, ... here is my take on it, as you said, you can lower the image size by compressing it before import inside the C3 project, (however see M2), as far as i know there is no way to alter the image format compression inside C3. you can convert from jpg to png etc lossless and stuff on export of frames inside the sprite.object animation but not on import, if there is a way i don't know it... and il be waiting also for a solution cause this might be something i missed.
M2: i don't think it matter if you convert before the image into a super compressed light0weight jpg-50 format, the moment it enters construct gets converted to jpg lossless or png 24 bit or something... so all your pre-load compression vanishes, not to mention that on export all those images you imported, get placed in 1 or more big image maps or sprite sheets, which has very poor compression, but which you can compress once you exported and have the file ... but you got to be careful at the dimensions and edges between images.
Edited: however, if the purpose of lowering the image is to do a low/high graphics specs, you can tweek the engine.render bilinear trilinear linear etc... i think in C2 there even was a plugin that was actually enabling webgl quality settings to go from high to low, and performance was insane and difference in quality of images not that big, but not sure if that plugin "whatever the name" (im missing it now), was converted, all i know is that was a commercial plugin for C2 that was adjusting the render quality and was the only plugin that was doing this.
an other solution to save memory or quality if that is the purpose, is redesign your game for max display 480p. if its a mobile intended app, and 720p for desktops under 2k, and 1024p for 2k and above.