Here is the original item:
With her heavy dictionary,
(A) Donna
thumps the top of her ancient computer
monitor. (B)
Whenever the color tint flickers
(C) from
normal to sickly yellow.
You wanted to do this:
With her heavy dictionary Donna thumps the
top of her ancient computer monitor. Whenever the color tint flickers from
normal to sickly yellow.
The comma after dictionary is correct. You must use a comma to attach an introductory prepositional phrase to the main clause that follows.
You might want to review the rules.