First, it is important that the OS your system originally came with since that would be the Image, if it were provided.
Also, support for Win 7 ends in January. That being the case, downloads, which were never easy to find, may have been removed.
Some newer Dell systems may not support Win 7. One the site I usually buy stuff, only Win 10 is available, not even Win 8.1.. Although you could probably find a copy somewhere...eBay seems to have it.. I am not recommending you go that way..