When using a bitmap image, there is the issue of “resolution” which refers to how many dots called “pixels” are being used. The more pixels, the better the clarity of the image and fewer pixels make for an image that has little stair-steps in it when you zoom in. One might think that more pixels are better. However the program has to do a lot of work while you are drawing your design and an image with a lot of pixels makes it take that much longer to do each step while you are working. So it makes sense to use an image that is not super-duper-hi-resolution.
When deciding what resolution you want to use, or how many pixels should be in the image, it is worth considering how that will match up to the embroidery design itself. In an embroidery design the most common density of a stitch is about 0.4mm or 4 stitch points. This equates to a resolution of 64 DPI (Dots Per Inch) and that means that a typical 4” (100mm) square design would only need an image of 250 pixels on each edge. The normal smartphone will give you thousands of pixels on an edge, so any photo today probably has more than enough resolution for what you need.
We recommend, for simplicity, that you keep your image to around 1000 pixels on either edge. You can go for more if you’re making a super large design, naturally, but otherwise this is a good number. A lot of graphics that exist on the internet are 1024x768 or 800x600 because these are common web sizes. And those images have plenty of resolution for you to create embroidery. If you are making a 14” design, you might want to bump that to 1500-2000 pixels on the widest edge. But remember more is not always better.