Microsoft "Office" Picture Manager which is fairly standard with most computers allows you to shrink the size as a percentage. So I open the photo in Picture manager ---- Edit Pictures------Resize -------- My pics are generally 2000-3000 so pic 20 or 30% or so until its a smaller file and resave as a different name so you don't lose the original. Many other graphics programs will allow you to resize. Or also crop all the rubbish out of the pic and you will have a smaller file
I do realise there is other advice on this subject posted, I'm sure peter will point you in that direction aswell
good luck rob