The only way how you can convert or export .sketch files to JPEG images, is to use the original Sketch software and no other. With the program users can save same parts or even whole Sketch drawings as .jpg pictures.
Convert Bohemian Coding Sketch drawing to JPEG bitmap image format.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert sketch to jpg on OS X
A professional design tool for OS X
Convert from sketch file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.