The online designer allows you to upload JPEG images to a template and modify it without needing any additional layout software.
However, when placing your JPEG image(s) you may see warnings that read the following-
First, don't panic! Your image will be converted to CMYK when your proof is generated. If you notice any odd color shifts in your proof, you may have to have someone with a program such as Photoshop edit the image for you.
The warning about your image not being 300 DPI can be a bit tricky- but if you see this warning and you know you have a good quality image, just proceed! If you find yourself having to scale the image down to fit the template quite a ways, the resolution should be fine (inversely, if the image comes into the template very small, there's a good chance that it is indeed low resolution).
Proceed with the rest of your design, and check your PDF proof files- those will let you know what you can expect on the final product.