Tokei

I think he wants to do this client-side only (without modifying the server files).
 
What you're looking for is normally quite simple, except in the particular case of lhz3 bosses (and a few others). Let's start with a basic mob sprite 'edit'.
 
To change the Pupa mob to Detale, you would do :
Find the real name of the mob (may have to use a database editor), in this case it's simply "pupa". This means the client will read and load the files "data\sprite\¸ó½ºÅÍ\pupa.act" and "data\sprite\¸ó½ºÅÍ\pupa.spr". You don't have to extract these. Find the real name of the mob you want to replace it with, detale in this case. Extract both "data\sprite\¸ó½ºÅÍ\detale.spr" and "data\sprite\¸ó½ºÅÍ\detale.act" and put these files in the directory "data\sprite\¸ó½ºÅÍ\" (I'm assuming your client reads the data folder first). If the folder doesn't exist within your RO folder, simply create it yourself... Rename the files to pupa.spr and pupa.act (btw, this is not a known trick, but if you select both files and rename the first one, it will rename both automatically and keep the original extension, which is quite useful and faster). When you go ingame, the pupa mob will appear as detale. Here's what you should get anyway : 

 
 
As for lhz3 bosses, these require more work. The regular mob uses the sprite name "SEYREN" and the boss sprite name is "B_SEYREN", which is being redirected to "SEYREN". Therefore both mobs use the same sprites.
 
This can be changed though! You will need to edit the file "data\lua files\datainfo\jobname.lub". The location of the file depends of your client setup, it may be already decompiled too... I can't go into full details for that part. You can download the file here though : http://svn6.assembla.com/svn/ClientSide/Lua_Project/lua%20files/datainfo/jobname.lua . You may have to change the extension to .lub depending of the client setup. Search for the line [jobtbl.JT_B_SEYREN] = "SEYREN". Change "SEYREN" for whatever you want, I'd put "B_SEYREN" to be consistent. Now extract any sprite you want, such as detale.spr/.act and change the name to b_seyren.act/.spr.

It seems complicated, but after a few times, it will be very easy . Hope this helps!
 
Edit : result ingame

All resources related to Violet Fear are in the rar file attached below. Have fun!
data.rar

There is already a script made for that, the file is named "magnifier.lua". Edit the file and change the "mag" value to your needs (mag = 2 means 200%).

Couldn't you just download jRO's GRF with a patcher (RSU)? Anyway, here they are (the patches are attached to this post) : 
For other jRO patches, you could just get the files from the links below.
Patch list : http://httppatch.ragnarokonline.jp/patch30/patchbbs/patch2.txt
Patches : ftp://ftppatch.ragnarokonline.jp/pub/dl-gunghoftp/roftp/
 
 ³²_¹ý»ç°ü      ³²_¹Ù´ÏÇìµåµå·¹½º     ³²_ÇǸ°Æ®½ºÄ«ÇÁ
Can be found in 2014-3-4Costama_Egg20.gpf
 
 
 ³²_Áúdz½Å·Ú    ³²_´õºíº»Ç︧    ³²_»çÀ̱׷é2
Can be found in 2014-2-4Costama_Egg19.gpf
 
 
 ³²_±â¿øÀǺ¢²É    ³²_ÃÊ¿øÀǹٶ÷    ³²_ÆĶõÀå¹ÌÀå½Ä
Can be found in 2014-4-1Costama_Egg21.gpf
2014-3-4Costama_Egg20.gpf
2014-2-4Costama_Egg19.gpf
2014-4-1Costama_Egg21.gpf

There you go :
 
jRO-Ragray patch file : http://httppatch.ragnarokonline.jp/patch30/patchbbs/patch2.txt
jRO-Ragray patches : ftp://ftppatch.ragnarokonline.jp/pub/dl-gunghoftp/roftp/
 
twRO-Ragray patch file : http://ronotice.gameflier.com/patch02/patch2.txt
twRO-Ragray patches : ftp://ropatch.gameflier.com/Patch/

See the attached file. You can use actOR with the magnifier.lua script (edit the 'mag' value) if you need to resize other sprites.
ae_moltres.rar

Sure, you could use the following script : 
for AID = 0, GetNumAct() - 1 do for FID = 0, GetNumFrame(AID) - 1 do for PID = 0, GetNumPat(AID, FID) - 1 do SetA(AID, FID, PID, 0x88); SetB(AID, FID, PID, 0x99); SetG(AID, FID, PID, 0x55); SetR(AID, FID, PID, 0xFF); end; end; end; Simply go in Script(S) > Edit... > copy and paste the above code and click on Run.

The client Kido made for you was using the "txt" files (idnum2itemdisplaynametable.txt, idnum2itemdesctable.txt, etc) to define your custom items. This 2013 client uses itemInfo.lua/itemInfo.lub instead.
 
I'd suggest you to read http://rathena.org/wiki/Custom_Items