Hey there!
The official
RPG Maker boards might be an easier place to get help (in general this place focuses on other programs and making games from scratch rather than RPG Maker), but I can give some pointers that should get you on the right track:
1. For changing an image you'll want to use a switch. To see a treasure chest event all set up with an example of switch usage, you can right click on your map (while in event mode) and select this option:
It will make a "treasure chest" event for you with two "pages", which are essentially states that an event can be in. Here, those states/pages are unopened and opened. Later pages will always take priority if the conditions for them are met- in this case, the switch for having opened the chest being activated. If you study the event generated by this action a bit I'm sure you will get it if it seems complex.
2. To make the player visibly carry something you would have two options: one would be making an edit of the character's graphic to be holding the object in some graphic editor (then hiding the object you just picked up with a switch), and the second would be programming a function into the engine with the RGSS scripting. However I do not know a lick of Ruby and wouldn't be able to help you there
also it would be overkill like reinventing the wheel compared to just editing the graphic.
Manipulating the position of an event with RPG Maker's event system would be a bit complicated though, it would involve some variable use to move the event in question to a particular spot, but it would still be comparatively faster/easier in the end then trying to code the function yourself into the RGSS.
3. Yup, this is simple! In events, go to page 2 and select "Set Move Route", then you can change the event targeted by the action with the dropdown in the top left.
Cheers, good luck and have fun making your game