Open your hack in Lunar Magic and click the Super (red) mushroom on the tool bar to extract the graphics (make sure that "Use Joined GFX Files" is selected under Options on the menu bar). This should create a folder called "Graphics" in the same folder as your hack. Inside the folder there should be a BIN file called AllGFX (if there isn't make sure that "Use Joined GFX Files" is selected and extract the GFX again). They're two ways of doing the following part. I'm posting the hard way because it works for absolutely any graphics in your hack and might be helpful to you later on.
--Easy Way (can only be used to edit some graphics in your hack)--
Go to a level in your hack that has the goal point question sphere in it and click the ? block on the tool bar that only has to top left quarter lit up (A.K.A. the 8x8 Graphics Editor) press down until you get to the last page. You should see the Egg graphics (in the top left) and the goal point question sphere graphics (near the middle). All you have to do now is right-click the egg graphic tiles and left click there you want to put them. Press F9 and save the GFX before closing the 8x8 tile editor. If this worked skip the next part of my post.
--Hard Way (but can be used to edit any graphics in your hack)--
You will need YY-CHR or Tile Layer Pro (graphic editing programs) which can be downloaded from the tools section of the site. I suggest Tile Layer Pro because it is what I use and in the next part of my post I will be referring to Tile Layer Pro.
Open Tile Layer Pro and go to the folder called "Graphics". Open AllGFX and you should get a bunch of weird green lines. In the menu bar go to View > Format > SNES.
The graphics should now appear properly and now all you have to do if find the graphics and change them. In case you need help with this too I have taken screenshots of where the graphics are located and what you have to do to change them.
1. Locate the egg graphics
2. Drag the graphics to the tile arranger
3. Locate the goal point question sphere graphics
4. Drag the graphics from the tile arranger to where the goal point question sphere is.
5. Go to "Save As" under File on the menu bar and double click AllGFX. It should ask you if you are sure you want to save, click yes.
--Final Steps (for both the easy and hard way)--
After you do either of those open your hack and click the 1up (green) mushroom on the tool bar. Unfortunately all of that only fufills half of your request. It might not be such a great idea to make it yellow because the goal point question sphere's palette is shared by other sprites. If you still want to make it yellow change these colours in the palette editor.
I think that there might be a way to work around that although unfortunately I don't really know how.
Hopefully that helps you.