Hi everyone,
I've just discovered this site and hope someone will be able to help me! I've been looking through the posts and I have a feeling I'm going to be here a lot

I want to upload some images to BigStockPhoto but the requirement is 25Mb or less. One particular image I'm trying to reduce is 34.8M, 4272 x 2848 pixels and 72dpi. I'd like to keep the pixel size as they say 4000 pixels sells more, although they accept 800. How do I reduce the file size? And am I able to increase the dpi?
Sorry if these are really basic questions! While I'm not new to photography, I've never really needed to know much about file types and so on.
Thanks for any help,
Vic