SQL Expression is one Feature in Crystal report,by SQL 
commands we pull the data from data base server to crystal 
report. 

Advantage:it's reduce the load on the server. bz we are not 
loading all the table into Crystal reports, just we are 
loading what we want using SQL.

Answer 1:
Yes,there is a way.User may create a word document,embed 
the required image as a picture object in it and then embed 
this word document inturn into Crystal reports(as a linked 
OLE object).

Any change in the word document would reflect in all he 
reports using this word doc.

Answer 2:
The following article would explain about this:
https://www.sdn.sap.com/irj/servlet/prt/portal/prtroot/com.s
ap.km.cm.docs/oss_notes_boj/sdn_oss_boj_erq/sap
(bD1lbiZjPTAwMQ==)/bc/bsp/spn/scn_bosap/notes%
7B6163636573733d36393736354636443646363436353344333933393338
323636393736354637333631373036453646373436353733354636453735
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%7D.do

Answer 3:
This was to confuse you further.There are formula 
fields,database fields,sql expression fields,paramter 
fileds etc and nothing like expression fields.

One feature is there in crystal report xi.First To insert 
an image we are doing Insert->click Picture -> we select a 
picture from a directory that you have access to and click 
on open and insert the picture where in report header 
section.
To make the above object dynamic we can follow below steps:
1.Right-click the static OLE object that you inserted, and 
choose Format
Graphic from the shortcut menu.
2. In the Format Editor, click the Picture tab, and then 
click the Conditional
Formula button adjacent to the Graphic Location label.
3. In the Formula Workshop, create the path
For example, if your path is stored in a database field, 
add that field to the
formula.
4. Click Save and close to return to the Format Editor.
5. Click OK to return to your report.
6. Click Refresh to update the reference link