We would recommend sending your artwork over at 300dpi and the correct dimensions you would like it to be printed.
If the artwork file you have is larger than you require, that is no problem at all! We can size the image down without losing quality.
However, if the image file is smaller than you require it to be printed then we will usually be unable to use it, there are exceptions though! Sometimes our artwork team can help you out with the design if it is simple enough to edit or recreate. For example, if your design is simply text then as long as we are told the font and it is free to download or you can provide the font file then we can recreate for you. For more complex designs we wouldn't be able to do this and would recommend you contact a designer to help you out.
If you are unsure that your artwork is correct or will work you can email us at firstname.lastname@example.org and we would be happy to help you!