If you are supplying your artwork as a .jpg file then we do really need it supplied at 300dpi (minimum) at the correct size. So an A5 flyer should be sent at 154x216mm (A5 plus 3mm bleed all round) at 300dpi.
This is relatively easy to do from packages such as the Adobe products and Microsoft Publisher, but not really possible from non-design packages like MS Word and Powerpoint, these would be much better being saved as PDF.
Although when creating files in Adobe Photoshop the original canvas must be set to 300dpi to begin with as it cannot be increased later, it requires a new file to be created. Please see our Photoshop page here.