Images have gone really small

Hi, on this site http://www.mpscarsalestyneside.co.uk/ the client uploads new images regularly on the home page - for some reason some they have started to get smaller and smaller.  I tried to upload one today - the original image was 500x500 - it uploads and shows fine, but then goes tiny on the web page. I haven't altered any coding on the website, can you please help?? Thanks.