When users look for Office document to image conversions, such as xlsx to jpeg conversion, they basically need one of two things possible. The most common thing would be to to simply get a picture of their document, which can be also done by print screen method or to export pictures saved inside the office documents (typically .docx, .pptx, .ppsx, .xlsx).
You may find utilities that have tools for both of these situations, but you can achieve the very same thing in MS Office. Making picture of your document can be easily done using PrintScreen key and then pasting the data to some painting / graphics program to crop and save it as image (usually JPEG or PNG). Specialized programs and converters of course make all this much easier and you will also get better results if you save your document as image in a program that offers this function, so you do not need to manually crop out the edges etc.
Graphs saved insides spreadsheets (.xlsx) are not pictures, they are actually XML generated. If you want to get them, you have to open the spreadsheet in Excel or similar program, select the graph and save / export that as a picture.
As for pictures saved inside documents. Latest XML based formats are basically renamed ZIPs and such you can rename the extension and extract the contents. Inside you will find a media folder which will contain the images you are looking for and you can simply copy them out.
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