If I read you question correctly, you want to insert a picture into a table
cell and have it match the cell exactly.
"I don't want the cell to change sizes" -
Table, Table Properties, Options button, uncheck "Automatically resize to
fit contents" box.
Cursor in the cell in question, Table, Table Properties, Cell tab, Options,
uncheck the "Same as the whole table" box, OK, click the Center option, OK.
Cursor still in the cell in question, Format, Paragraph, Indents and Spacing
tab, ensure that all indents and spacing are 0, Special is (none), and Line
Spacing is single.
Now when you insert your picture (Insert, Picture, From File) is should
match as close as Word is capable to the width of the cell.
If you need more precision, a floating image adjusted manually could work,
fiddly and it may not stay with the cell very well if the table is moved by
edits before it in the document.
Hope this helps
DeanH