The problem is that you can't directly mix field transclusion syntax, e.g., {{!!fieldname}}, within other wiki syntax, e.g., [img[...]]
The solution is to define a macro that constructs the desired wiki image syntax, like this:
\define showImage() [img[images/$(currentTiddler)$.jpg]]
Then, invoke it like this:
<<showImage>>
Notes:
* By default, within each tiddler, the variable "currentTiddler" holds the name of the tiddler
* In normal tiddler content, you can use either {{!!title}} or <<currentTiddler>> to display the name of the tiddler
* Within the macro definition, $(...)$ does a text substitution of the indicated variable, where the variable is defined outside the macro
* After variable substitutions are done, the resulting macro content is "returned" for rendering
* Thus, if the above macro is invoked within a tiddler named "Tiddler1", the macro content returned by <<showImage>> will be: [img[images/Tiddler1.jpg]], which is the syntax you want.
* To use <<showImage>> in multiple tiddlers, just place the showImage() macro definition in a tiddler by itself (e.g., "ShowImageMacro"), and then tag that tiddler with $:/tags/Macro.
Also, as an alternative to using a macro definition to construct the desired syntax, you could use the <$image> widget with a parameter value that is assembled using "filtered transclusion", like this:
<$image source={{{ [[images/]addsuffix<currentTiddler>addsuffix[.jpg]] }}} />
However, this syntax is obviously more complex to produce the exact same result.
enjoy,
-e