This topic has been discussed many times before. Lots of details on the web about the Pros and Cons of JPEG images, GIFs and PNG graphics.
Reducing resolution is not the only way to reduce file sizes, reducing image quality (ie increasing compression) saves space too. Most photos compress very well and look OK at 20% quality. Google search for JPEG, GIF, or PNG, or search this site for short answers, e.g.
It depends on the Photo Imaging software you use, the popular ones such as Paint Shop Pro have the option to alter the amount of compression under options, Click "File", "Save As", choose the type of image to save (.jpg) and then somewhere in the save window there should be "Options". The amount of compression to choose should be there. I have looked at the 3 photo progs I have on this machine and, whilst looking a Little different to each other they all do the same thing.
Failing that, use a 3rd party programme, look at google with words such as "jpeg optimizer". This will give hundreds of responses - at around $15 upwards, some may be free.