Tecplot can export PNG and JPEG images, but for animations it can only export AVI, Raster Metafile, and FLASH. If you want to export a series of PNG or JPEG images, you can do this, but it takes a macro.
It is not a difficult macro to write - all you need to do is record yourself (Scripting/Record Macro) setting up the first image you want to export, and exporting the image (in JPEG or PNG).
Then you can open the macro in a text editor and in a few minutes edit it so that it will repeat these steps, for all of your timesteps.
We have a reference manual most of which is devoted to the macro language. It contains a list of all the macro commands Tecplot recognizes and their syntax. Here is a link to where you can download it.
The document on that page that you want is the Tecplo0t 360 Scripting Guide. it is in the left most column, third from top.