Datatypes with os3.1

[amiga4001]

I installed 3.1 on my 1200 and with it toolmanager and newicons.
I wanted to put a picture in the background.
Just a simple cartoonwallpaper as .iff with not too much colours as the 1200 has only a 030.
Downloaded some pictures on the pegasos and converted them to .iff and resized them to the amigascreensize(640x512).
But amiga won't display them not as iff or as original jpg.
Why??I thought it had these datatypes in os 3.1?
Or am I missing something?

[amiga4001]

Thanks for the help.
I installed the picture datatype from P96.
Also installed newer ilbm datatype(24 bit support) and I installed the ak datatypes.
All pictures are shown now.
Now I must convert the pictures from jpg to 8bit's iff.
Think this is fastest way to show on the 1200?
I have some graphical programs but no experience in this.
I have Adpro which I do a exute and the a render to raw after this it allows me to save it as 24bit or 8 bit.
But imagecouring is very poor doesn't look like 256 colours actually.
Also have FXpaint iff it can perform such actions.
Or is there a nice aminet tool for this?