wp garage theme problem

Hi! I've already faced such problem: I am working on garage template (http://wp24h.com/garage-wordpress-theme.html) and wanted to replace graphics placed in wp-content/themes/Garage/images(top.jpg; footer.png etc.) I just uploaded my own files having the same pixel sizes as original ones and renamed them. The problem is that it doesn't work at all. Moreover, using ftp client i can have an outlook but when original files are downloaded I can't open them (the message is files might be damaged, broken or too big. What may i do wrong? Looking Forward to hearing from you guys! :)