The Photo slide support of image types is not documented, but uiTLM has successfully test these types:

Customization features available in the widgets Edit dialog are:

• Photo folder - User-selectable folder on the device
• Change interval (sec) - Image will sequence within a user-selected range of 6 seconds to 60 seconds
• Skin - The skin is the actual viewing area of the container making this option questionable. A best use is with a highly transparent color or shape with color to give the image a consistent or gradient hue. The Icon pack option could be used as a “logo” but any insert will be centered and is not movable.
• Rounded corner radius - The widget's container defaults to a square but can be resized by tap-hold and grab. The user can choose to have corners rounded by selecting the desired radius with a range of 0 (square) to 300 (circle). Again diameter can be adjusted, even making an ellipse.
• Disable 3D animation - This option disables the default presentation of images in 3D animation consisting of randomly selected launcher animations such as cube, sliding (4 directions), flip and more. Images will only change by sliding in four random directions.
• Keep photo centered - This keeps all images (some peripheral objects may not be shown) while the default shows images, larger than the widget's container, in a pan-type view roaming the full image in a random manner.
• Random pick - Presentation of images defaults to the directory's alpha-numerical order. Activating this open will cause random viewing.
• Grayscale - This option, if activated, displays each image as a gray-scaled image.
• Background - This offers the generic Background options of the launcher which is of questionable use since the image itself occupies the full container. However, the use of Color in a transparency format (where A of ARGB is 00) may provide an acceptable tint to the shown images. See here.
• Margins: Allows adjustment of the margins surrounding the widget in its container. See here.