A JPG image is a raster image. This means the image consists of individual color dots or pixels. E.g. a raster image might be 600 pixels from left to right and 300 pixels from top to bottom.
DPI means "dots per inch" and refers to printing resolution, i.e. the number of image pixels printed into one inch on the printout. DPI is not a property of the image file (at least not in the strict sense, there might be resolution hints in the image meta data).
Any (raster) image may be printed at any printing resolution. As such, any JPG image exported from yEd may be printed at "300 DPI or higher". However, the number of pixels in the image and the printing resolution determine the actual size of the printed image on the printout. E.g. printing an 600x300 pixel image at 300 DPI will result in a 2 inch by 1 inch image on the printout. At 600 DPI printing resolution, the same image would be 1 inch by 0.5 inch in size on the printout.
Thus the way to prepare an image for printing is the following:
1. Determine the actual printing resolution.
2. Decide on the desired width and height (in inch) of the image on the printout.
3. Calculate the required number of pixel for the image. (DPI times width in inch yields the required number of pixels from left to right, DPI times height in inch yields the required number of pixels from top to bottom.)
It is possible to export images with a specific size from yEd - however, it is cumbersome to do so. Usually, it is easier to export from yEd with content in mind (instead of required size) and then prepare the image for printing in an image editing application such as the GIMP or Adobe Photoshop.
Sophisticated image editing applications (such as the aforementioned ones) also offer the possibility to specify the desired printing resolution of images in the image's meta data (which yEd does not). Such printing resolution hints are often used by text editing software such as Microsoft Word or OpenOffice Writer when embedding images into text documents.
