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))))))