Pixlr.com is a free online bitmap editor that you can use through your browser, no need to download or install anything. Pixlr will let you choose an image from a file or a URL to upload into their image editor. From there you can resize the image, then save it as a bitmap.
Patterns must be saved as bitmap (.bmp) files to be able to be displayed on the hoop. There are several tools you can use to save an image file as a bitmap file.
To save a pattern in Pixlr, check out this Help article: How do you save an image as a bitmap in Pixlr?
To decide the best size of the pattern for your hoop, check out this Help article: Pattern Sizing for bitmaps to fit your hoop size
For how to resize a pattern/bitmap/image in Pixlr, check out this video: