Do the calendar people advise the ideal file sizes (pixels per inch. and physical size, see attached) and whether CMYK or RGB?:
For litho print, 300 pixels per inch is the minimum, but I suspect this is digital CMYK.
Thanks Chris - there is no specific technical guideline supplied - although I'm sure they would tell me if I asked. What happens is that you download a picture to be used and then try to slot it into the calendar month template , and if the picture file is too small it will flag a message - effectively rejecting the image.

Now that I know picture files can be "boosted" I can re-look at some of the pictures I've already got and forward them to you .Since you first suggested it - for the sake of consistency I'll send them to you (Jim hope you don't mind !)