Most drawing programs and some page layout programs can save in JPG format
If the image is purely photo or similar a resolution of at least 200 dpi at the required finished size is best. If the image contains text or line drawings, then these file formats are not ideal but if they have to be used, choose a resolution as high as possible (the file size could be very large). JPEG files are compressed and the quality can sometimes be very poor.
Internet images are usually 72dpi and often contain compression artefacts. This doesn't matter for computer screens but they are often not suitable for printing unless the image is very small or the quality is unimportant.
Large format printing is particularly susceptible to poor quality images because all the defects are greatly magnified.