Opening the JPEG and doing a Save As in Photoshop is not a good idea.
First, a JPEG is a JPEG is a JPEG is a JPEG. There is no such thing as a "Mac JPEG" or a "Windows JPEG." All JPEG files follow the same standard; you are not 'converting" the JPEG somehow by opening and re-saving it.
Worse, though, is the fact that doing this damages the quality of the picture. The JPEG file format uses "lossy" compression--that is, it deliberately degrades the quality of the picture to make the file size smaller on disk. The JPEG format was invented for situations where file size on disk is critically important but image quality is not important. Because JPEG uses lossy compression, every time you open a JPEG and re-save it, the quality goes down. If you open it again and re-save it again, the quality goes down again.
999,999 times out of a million, when a Windows user can not accept a JPEG file, it is an error in the email settings.
The error usually has to do with the "AppleDouble" setting. AppleDouble was invented because Mac files have two "parts," which are called a "resource fork" and a "data fork," and Mac files also contain extra information stored in the disk directory called "Finder metadata" (which includes the icon of the file, the file Type and Creator codes, and so on).
AppleDouble works by taking the file, breaking it into TWO files (one for the data fork, one for the resource fork plus Finder metadata), and sending two attachments. Windows users often become confused and frustrated when they see a file that has two attachments. One of the two attachments is the JPEG file itself; the other one is worthless on a PC, because it contains only Mac icon information, Mac creator and type codes, and so on.
Windows users who receive a file sent this way will often try to download one of the two attachments they see, then not be able to open it (because the part the download is the resource part), then get frustrated and give up. I have actually stood over the shoulder of Windows users who receive a file in AppleDouble format. They will download the resource part, discover that they can not open it, and quit--without ever trying to download the other file attachment!
The safest way to send a JPEG to a Windows user is to use Windows encoding, not AppleDouble. In Apple's mail program, there is a checkbox hat is labelled "send windows friendly attachments when you attach a file. In other programs, it is usually somewhere in the Preferences command, and it may be called something like "Use Windows (base64) encoding" or "attach files using base64" or something like that.
Because a JPEG file does not need or use a resource fork, it is safe to send a JPEG file this way.
AOL is a spacial case. AOL users can not accept emails that have more than one file attachment under any circumstances, unless they are smart enough to find and use a third-party "MIME decoder" program. If you send a file to an AOL user using AppleDouble, the AOL user can not use it, because AppleDouble always sends two file attachments and AOL users can only accept email with one file attachment. Likewise, if you send several files to an AOL user in the same email, the AOL user can not accept it. With AOL, you must always send files WITHOUT AppleDouble (even to Mac AOL users!), and you must send only one file per email--if you want to send three JPEG files, you have to send three emails.
