This is quite possibly not the a fault of ACDSee, as this problem is common to Internet Explorer users, as documented by Microsoft in many places, including these three tech support manuals:

• Internet Explorer Saves Images as Bitmaps
• Right-Clicking, Selecting 'save Picture As' Does Not Save Image with Correct Extension
• Internet Explorer does not save graphics files in the proper format

In a nutshell, though, what may of happened is that a damaged program file such as an ActiveX or Java object has been downloaded, mucking things up.

To fix it you will need to clear your Tempoary Internet files and remove any damaged or unknown files from your Downloaded Program Files folder.

Here's what you need to do:

When using Internet explorer choose Tools --> Internet Options

Under "Temporary Internet files", click Delete Files.

Click OK on the next pop up window,

Then select the Settings button next to the Delete Files button

In the following Settings window click the View Objects button

In the next window find the Status column for the list of items Check for any items marked Damaged Or Unknown and delete them by right clicking on them (in the Program file column) and select Remove.

Click Yes when asked whether you're sure.

Close the window and click OK on the remaining windows that you have opened.

Now quit the program, restart it, and try saving a graphic image or two.

Hopefully that should do the trick.