I've used IRFANVIEW to do that but before we go further, I didn't read if you knew you would lose resolution or detail in the images.
As to your GIMP question, I used it long ago and found that most folk cringed at the learning curve. Great stuff but for this, why not IRFANVIEW? As tutorials and more are on the web I'll stop here.
I have some images taken by someone's mobile phone, some of which are large eg. 5MB+ each.
I want to lower the size of these to something in the small kilobytes size to send to others. There are only a small number of images to be treated.
I haven't downloaded Gimp yet. But wanted to know;
i) if lowering image resolution is something that Gimp can do (I assume so)?
ii) I don't really use such software much, but how does Gimp compare to Paint as a replacement?