[PinkMilk]

Replacement:

<img src="{param}" style="width:{option}px;height:auto;" />

JSFiddle height:auto demo; height:auto will allow you keep aspect ratio needing only width to be adjusted.

If adjusting both width and height is really necessary try this mod:
Multiple Options in Custom BBCode

with this replacement:

<img src="{param}" style="width:{option1}px;height:{option2}px;" />

Members would then simply add highlighted text in option popup:

[somebbcode=100;50]image/link/here[/somebbcode]

Update:
Just noticed this request is for vb5 and mod mentioned above is for vb3, will leave post however for vb3 users that might be interested.