With Access, the most efficient way to store pictures (and for SQL Server as a remote back end, this is even more true) is to store the fully qualified file spec to the picture file, which will of course be expressed as a text string. Let Access worry about the graphics in the front end file.
For each image control on a form or report, there is a .Picture property. To put a picture in the image control, load that device:/path/name.type string (or the UNC version, \\server\path...path\name.type) to .Picture and do a form .Refresh to get it displayed.
For the upload you mentioned, you can create some VBA code that would be launched by a button-click. You can create a File System Object and ask it to trigger a file-picker dialog.
Once you have an FSO declared, you pick the file using the picker and then can use some simple string parsing to split out the path from the file name and type. Some of the methods of the FSO will do part or all of this parsing for you. (Hint: Do a Web lookup for File System Object and check out the stuff from the MSDN site.)
Next you generate the fully qualified file spec for the server folder based on the name you just parsed and the path to that hosted folder. In either case, the .Connect string for the back-end tables will contain the information about the host server.
Having both the original spec (from the picker) and the new spec (formed by some parsing) in string variables, you can easily use the FSO to launch a .FileCopy method to make a copy of the local file to the shared folder on your server.
Finally, you can store the new file spec as text in your DB.
Now, for this to be set up correctly in a form, you need the form to be bound and you can then have the FormCurrent routine load up whatever it needs to load up.
Note that you might have to look into condition formatting issues when using a continuous form. I have not tried something like you are trying so you might need to research that. Search this forum for "conditional formatting" as a way to find discussions on the method you would need to use. If I recall correctly, if you DON'T pay attention to the conditional formatting issues, then the last image you load is the only image you see because all continuous forms will use the last picture. But there are ways around that result, I just have not tried them because I never needed to do so.