When I try to add an image to a K2 item, the upload fails when I try to upload an .jpg image with the size of about 6MB.
It works when I upload a small .png image, but when I convert my 6MB .jpg image to a .png file, then it has about 20MB and that is just too big.
Has anyone an idea what could cause the problem with the pictures?

PS: This is the screenshot with the errormessage I get when I try to upload a .jpg file - imgur.com/UOso4Zv

Did you try to upload a small size jpg image, less than 2MB?

Most servers have a 2mb limit, this means that the image will never be uploaded. This being said even if the limit is larger it would be still be very intensive for most servers to resize 6mb photos.
You are highly advised to use images less than 2mb, ideally round 500k