GIMP is an outstanding program but (because it aims to provide everything that a full-time professional image-manipulator might require) it's got a
very steep learning curve and it's easy to be baffled by it. (I'm experienced with a 'professional' version of Photoshop but, while I love GIMP, I quite often have to go through some head-scratching to work out the best way to use it).
Serif PhotoPlus might be easier to use:
http://www.serif.com/free-photo-editing-software/?MC=FSSPHOTOPLUS
Almost all image-processing programs will have a 'Sharpen' option, that can help a little (but at the cost of introducing 'grain' into the picture). Some might also have further options, such as 'Sharpen Edges', which seeks to avoid such grain being added other than around the edges of objects. However the results are never really outstanding.
To deal with over-exposure, all relevant programs offer 'brightness' and 'contrast' controls but (where available) you can usually get better results by tweaking the 'gamma' setting and adjusting the shape of the graph within 'curves'.
Lastly, with regard to creating a collage, it can sometimes be easiest to forget about using photo-manipulation programs (having already used them to 'tweak' your images anyway) and instead use a desktop publishing program to position and resize your images. This would meet your needs easily:
http://www.serif.com/desktop-publishing-software/?mc=FSSPAGEPLUS