Converting and uploading of pictures to web gallery

I'm building a web page with user area, where registered users will have a possibility to upload / delete pictures to / from their gallery, so other users can see it.

I tried to find some solution which would allow me to convert/resize images (I tried to find also FaceBook solution), unfortunately I had no luck or converter didn't work well. I don't want to restrict users with size of images. It means, users do not need to care if their pictures will have 5MB each or just 200KB. All files with width size > 1200px would be converted to width 1200, smaller sizes will be kept as they are. In addition, I will need also thumbs not to load images in their full (1200px) size.

Can you please advise?

Thanks! Jan