It may be that the application (RED) is displaying the wrong error message for this problem.
Does the name of the file that you are trying to import include the .jpg suffix? If not you might try changing that and reattempting to import the image. How big is the image (pixel dimensions?) Are you on Win or Mac? Also, are you using the 32 bit or the 64 bit version of RED 5?
I had a similar issue when i wanted to open an image in RED.
I found out that RED doesn't support non english letters (é) in my case.
If I change it to this letter by a "e", no problem. It might solve your problem. Let us know
I reported this issue to tech support many months ago, but nothing has been done.
Thank you Guillaume,
you are very kind but the name of my image is "IMG_5765.JPG".
The problem is not solved at the moment like also another problem I had some weeks ago with paint into particles system of RED.