You probably want something like this. I think you need a function that matches the img, so you can get the region and remove flyspell overlays. Also you need to modify the add-hook call a bit.
(setq x-gtk-use-system-tooltips nil)
(defvar image-tooltip-re (concat "\\\\includegraphics{\\(?1:.*\\."
(regexp-opt '("png" "PNG" "JPG" "jpeg"
"jpg" "JPEG" "eps" "EPS" "pdf"))
"\\)}")
"Regexp to match included images")
(defun image-tooltip (window object position)
(save-excursion
(goto-char position)
(let (beg end imgfile img s)
(while (not (looking-at image-tooltip-re))
(forward-char -1))
(setq imgfile (match-string-no-properties 1))
(if (file-exists-p imgfile)
(progn
(setq img (create-image (expand-file-name imgfile)
'imagemagick nil :width 800))
(message (propertize "Look in the minibuffer"
'display img)))
"No file found"))))
(defun img-match (limit)
"Match bar in at the end of a word."
(when (re-search-forward image-tooltip-re limit t)
(flyspell-delete-region-overlays (match-beginning 1)
(match-end 1))
t))
(add-hook 'LaTeX-mode-hook
(lambda ()
(font-lock-add-keywords
nil
'((img-match 1 '(face font-lock-keyword-face
help-echo image-tooltip))))))