Dale, your short questions don't have short answers.
If you need help figuring out how to prepare an image for uploading, I suggest you help those who will be helping you by identifying what you do know to make it easier to help with what you don't know.
For instance, you do know that you need to upload an image that can be viewed on the web, so what are your intentions, do you plan to create a .jpg file from your design?
What programs do you have? Will you be designing in Illustrator and exporting as a .jpg
Or do you have Photoshop? Will you be able to open your design in photoshop?
If you can open in photoshop, you can pull down the "image" menu & choose "image size" from there you can specify 72 dpi AND you can specify the height & width in PIXELS. Don't worry about inches, you need a rastor image made of pixels, so you should be able to identify the pixel dimensions.
If you don't have the programs or the knowledge to check your pixel dimension, but you do have a .jpg file, you can look below the box where you enter a reply & click on the "manage attachments" button. This is where you can upload an image to signs101 directly from your computer. If it is too large, a box will pop up & notify you what dimension you tried to upload. This would help you determin if you should shrink it 75%, or 50% before you try again.