I'm a mile out of my depth on the programmy stuff & have not researched or done the sort of trips to give you meaningful comparisons to similar sites, but just to pick up on
@Octet image suggestions. Assuming photoshop: The file drop-down menu on the top bar, there is a 'save for web' (*) option to pretty well 1-click optimise your pics for internet display.
(*) newer Photoshop versions will call it 'save for web & devices'.
Internet bog standard images publish at 72ppi. Higher resolutions make for bigger, slower loading files & are aimed at different markets/requirements than 'normal' webpages.
Most cameras default shooting resolution is 300ppi, better to keep this or better and downscale in the computer than photograph in fax machine quality at the outset, that way you have your master copy archive of good quality images for your own enjoyment & should you require for physical display & advertising, magazine or brochure publishing etc, along with the lesser quality working copy web ones that will load fast and look good enough on your average punters computer screen.
Its a long time since I touched GIMP etc now but I would assume they all have a similar web optimisation save option.
You can do it manually too, or use this to assess the physical size and resolution of the pictures you're working with, Giving Photoshop as the guide again: Top menu bar: Image : Image Size & the info pops up in its own new window you can adjust and interact with until you're happy. Then 'save-as' & its options pop up, as you would do with any other sort of file to ensure you keep your original master intact & have your web optimised duplicate.
My apologies if this is teaching granny to suck eggs, but just picking up on the image format observations.