I'm using Slackware linux.
I'm not having clear display.

It's impossible for anyone to help you based on eleven words. If you want us to point you in the right direction, you'll need to explain your problem in vastly greater detail, as well as whatever you've tried so far.

when I open image file, it appears as the colours are disturbed.
Lots of dots can be seen, as if each part of the image is separated.
And when I open mozilla firefox, It opens with different colours.

I'll make a wild guess and say that you haven't configured your videocard and are still using vesa. Probably, the vga mode isn't set correctly too.
You need to configure your X server. What videocard do you have? Look at the output of
/sbin/lspci

00:00.0 Host bridge: Intel Corporation 82810E DC-133 (GMCH) Graphics Memory Controller Hub (rev 03)
00:01.0 VGA compatible controller: Intel Corporation 82810E DC-133 (CGC) Chipset Graphics Controller (rev 03)
00:1e.0 PCI bridge: Intel Corporation 82801AA PCI Bridge (rev 02)
00:1f.0 ISA bridge: Intel Corporation 82801AA ISA Bridge (LPC) (rev 02)
00:1f.1 IDE interface: Intel Corporation 82801AA IDE Controller (rev 02)
00:1f.2 USB Controller: Intel Corporation 82801AA USB Controller (rev 02)
00:1f.3 SMBus: Intel Corporation 82801AA SMBus Controller (rev 02)
00:1f.5 Multimedia audio controller: Intel Corporation 82801AA AC'97 Audio Controller (rev 02)
01:05.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 10)
01:0a.0 Modem: Smart Link Ltd. SmartPCI2800 V.92 PCI Soft DFT (rev 02)

OK, now that you know what it is, look inside /etc/X11/xorg.conf file and see what is listed as the "Driver" in the "Device" section. In fact, there are thre Device sections there, one will list "vga" as the driver, the second will say "vesa", and the third one should be something like "i810" for your Intel card.

If it's not there, run the xorgconfig command from a root terminal. You'll answer a couple dozen questions and your X will be configured.

# **********************************************************************
# Graphics device section
# **********************************************************************

# Any number of graphics device sections may be present

# Standard VGA Device:

Section "Device"
Identifier "Standard VGA"
VendorName "Unknown"
BoardName "Unknown"

# The chipset line is optional in most cases. It can be used to override
# the driver's chipset detection, and should not normally be specified.

# Chipset "generic"

# The Driver line must be present. When using run-time loadable driver
# modules, this line instructs the server to load the specified driver
# module. Even when not using loadable driver modules, this line
# indicates which driver should interpret the information in this section.

Driver "vga"
# The BusID line is used to specify which of possibly multiple devices
# this section is intended for. When this line isn't present, a device
# section can only match up with the primary video device. For PCI
# devices a line like the following could be used. This line should not
# normally be included unless there is more than one video device
# intalled.

# BusID "PCI:0:10:0"

# VideoRam 256

# Clocks 25.2 28.3

EndSection

# Device configured by xorgconfig:

Section "Device"
Identifier "** Intel i810 (generic) [i810]"
Driver "i810"
#VideoRam 32768
# Insert Clocks lines here if appropriate
EndSection

As Uncle_Theodore suggested, as root, run:
Or, if you want something easier, run:

Solved the problem, but the resolution has been changed to 800 X 600

I want to use 1024 X 768. Is it possible?

Finally solved the display problem.
I'm using 1024 X 768 resolution.

xorgconfig helped me