By default, theimagefile uses browser cookies to remember the account under which you are logged in.
Cookies are typically shared among all different windows of the same browser. So, if you have five Internet Explorer windows open, and in the fifth window you login as a different user, then all five browser windows may become logged in as that user.
Cookies are not shared between different types of browsers, however. If you have one Internet Explorer window and one Firefox window, each could be logged in to the same site under a different user account.
So, to sign in to theimagefile with two different accounts at the same time, use one of these techniques:
Use two different browsers, side by side (recommended)
Use two different browsers, side by side
In this model, you simply open your two favourite browsers, say Internet Explorer and Firefox. The first browser is always used for your primary account (i.e., a photographer account) and the second window hosts your secondary account (i.e., a free account for a test image buyer).
It takes a little getting used to, but this simple solution works great. The slightly different interfaces of the different browsers will act as a subtle cue to remind you of which account you are using, and the specialised role of that account (i.e., photographer or buyer).
Disable cookie authentication on secondary account
One way to avoid cookies, of course, is to just disable cookies in your browser. theimagefile will auto-detect this, and automatically start using the "sid" parameter.
Another way is to login and go to "Control Panel" > "My Profile" > "Preferences" => "advanced session preferences". Under the section "How to Store Session Identifier", choose "URL Only". Scroll down and choose "Save Data". Now that account will always store the session in the URL instead of the cookie, even if your browser supports cookies.
How to transfer files between two accounts
To transfer files between two accounts first either move or copy all files or collections to a new folder in your Corporate collections area of the donating account and give access to this folder by adding the email address of the receiving account to the collection access rights (Access rights on Corporate Collections). Then log in to the receiving account and create a folder ready to import the files into. Once created go to the bottom of the My Images page and select the donating account folder from the drop down menu and click "View". From here you can download all or selected files to your receiving collection.